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Tumor cells engineered to express a bacterial flagellin protein could represent a new vaccination strategy against cancer, researchers suggest. Flagellin is a known toll-like receptor 5 (TLR-5) ligand, and is also recognized by NLRC4 and NAIP5 (neuronal apoptisis inhibitor protein), two members of the Nod-like receptor (NLR) family. A team led by Julie Magarian Blander, M.D., at Mount Sinai School of Medicine, demonstrated that injecting mice with a cancer cell line engineered to express the model antigen ovalbumin fused to flagellin effectively immunizes the animals against developing cancer when they are subsequently given a subcutaneous injection of live parental tumor cells.

Importantly, the researchers showed that this protective effect hinged on simultaneous engagement of TLR5 and NLRC4/NAIP5 by flagellin: NLR engagement was critical to the animals mounting lasting T cell anticancer immune responses. The team reports in Science Translational Medicine, in a paper titled “Simultaneous Targeting of Toll- and Nod-Like Receptors Induces Effective Tumor-Specific Immune Responses.”

Dr. Blander et al. utilized an EL4 thyoma cell line engineered to express a fusion protein comprising the model antigen ovalbumin (OVA) and flagellin from Salmonella typhimurium. Initial tests demonstrated that injecting these cells into the peritoneal cavities of experimental mice triggered the recruitment of neutrophils and macrophages that rapidly cleared the cells. This cell removal response was dependent specifically on TLR5 signaling, but analyses also indicated that both TLR5 and NLRC4/NAIP5 signaling were equally important for priming antitumor CD4+ and CD8+ T cells and suppressing tumor growth.

Critically, when engineered OVA-flagellin-expressing cells were used as a vaccine, the immunized animals were able to fight off tumor development when subsequently challenged with live parental cancer cells, as long as neither TLR5 nor NLRC4/NAIP5 recognition was compromised. This protective effect was also evident when tumor cells expressing just flagellin were used for immunization.

Conversely, protection wasn’t afforded by vaccinating the animals with cancer cells expressing just OVA, even when these cells were injected in combination with a recombinant flagellin protein. Effectively, triggering CD4+ and CD8+ responses relied on the associative recognition of flagellin and the tumor antigen by dendritic cells to enable optimal antigen presentation to T cells and meaningful T cell responses. “Enhanced antigen presentation relied primarily on the physical presence of flagellin within tumor cells,” the authors stress.

Dr. Blander’s team says their findings suggest that it may be possible to generate a human anticancer vaccine based on flagellin-modified tumor cells, even without the need to incorporate tumor-specific or foreign antigens.

“Our findings delineate a new strategy to induce anticancer immune responses consisting of introducing microbial structures with dual TLR and NLR stimulatory activity into tumor cells,” the authors conclude. “This ensures recognition of tumor-derived antigen within the inflammatory context of microbial recognition and additionally activates both the phagocytic and the cytosolic pathways of innate immune defense against the tumor.”

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